1. Department of Medicinal Chemistry, Royal Danish School of Pharmacy, Universitetsparken 2, DK 2100 Copenhagen, Denmark, Department for Molecular and Structural Biology, University of Aarhus, DK-8000 Aarhus, Denmark, Department of Biochemistry and Molecular Biophysics, Columbia University, 650 W. 168th Street, New York, New York 10032, and Howard Hughes Medical Institute, Columbia University, New York, New York 10032